A heat-stable inhibitor of low Km phosphodiesterase, present in extracts of epididymis and prostate of normal adult rats, was lost from these tissues after castration or treatment of the animals with antiandrogens, but was maintained by injections of testosterone propionate. The inhibitor, extracted from accessory glands of normal rats inhibited in vitro the phosphodiesterase activity of accessory glands of castrated animals. These findings suggest a mechanism of hormonal control of cyclic AMP-dependent functions by the local induction of an inhibitor of phosphodiesterase by androgen in its target tissues. In the proposed study the preliminary findings will be extended; the inhibitor and activator will be isolated and characterized and the molecular mechanism of the action of androgen will be investigated.